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LIFE SCIENCES STUDY GUIDE
Grade 12
A member of the FUTURELEARN group
Life Sciences Study guide
1812-E-LIS-SG01
Í2,È-E-LIS-SG01_Î
Grade 12
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ALL RIGHTS RESERVED ©COPYRIGHT BY THE AUTHORS The whole or any part of this publication may not be reproduced or transmitted in any form or by any means without permission in writing from the publisher. This includes electronic or mechanical, including photocopying, recording, or any information storage and retrieval system. Every effort has been made to obtain copyright of all printed aspects of this publication. However, if material requiring copyright has unwittingly been used, the copyrighter is requested to bring the matter to the attention of the publisher so that the due acknowledgement can be made by the author.
Life Sciences Textbook & Workbook Grade 12 NCAPS ISBN-13:
Printed 9781869218812 E-pub 9781869218942 PDF 9781869218928
Product Code:
LFS 72
Authors:
Jen Grogan Ruth Suter
DTP: Graphics: Cartoons:
Kerry Thomas © Kerry Thomas © Tony Grogan
First Edition:
August 2015 (Revisions based on Examination Guidelines - Grade 12, from 2014)
Second Edition:
May 2016 (Minor revisions, includes addendum)
PUBLISHERS
ALLCOPY PUBLISHERS P.O. Box 963 Sanlamhof, 7532
Tel: (021) 945-4111, Fax: (021) 945-4118 Email: info@allcopypublishers.co.za Website: www.allcopypublishers.co.za
i
INFORMATION SHEET LIFE SCIENCES GRADE 12 NCAPS ADVANTAGES of the TEXTBOOK & WORKBOOK The information is current and has been updated. The lay-out is user-friendly with clear, logical and informative diagrams. Important key words are printed in bold throughout the book. The book includes the following: • Learning activities These are for consolidation and homework, marks are included. They include a variety of questions, e.g. essays, short questions, multiple choice, mix and match, most of which are answered in spaces in the textbook. • Investigations and experiments • Projects • Dissections • Additional information: Did you know? This is unusual or interesting information. Something extra This is extra information, to challenge the interested learners.
ENDORSEMENT “Abbotts College is renowned for the results achieved by its Matric students. Jen Grogan and Ruth Suter are two of the most successful teachers who have ever taught at the College and their legacy lives on; not only in the lasting example that they set, but also in the fact that the textbooks that they have written remain the foundation for our teaching. We recommend their work unreservedly, the proof of which lies in the continued excellent results achieved by our students.” Malcolm Law (ex National Principal, Abbotts Colleges)
The AUTHORS Jen Grogan B.Sc., U.E.D.; 28 years of teaching experience at Grade 12 level at Victoria Girl’s High (Grahamstown), Alexander Road High School (Port Elizabeth) and Abbotts College (Cape Town); co-author of Life Sciences - Textbook and Workbook Grade 11 (2010) & Grade 12 (2011). Ruth Suter B.Sc., STD; 33 years of teaching experience at Grade 12 level at Westerford High School and Abbotts College (Cape Town); co-author of Life Sciences - Textbook and Workbook Grade 11 (2010) & Grade 12 (2011).
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LIFE SCIENCES TEXTBOOK AND WORKBOOK GRADE 12 NCAPS CONTENTS
Page no.
1.1
DNA: Code of life
1
1.2
Meiosis
19
1.3
Genetics and Inheritance
35
2.1
Vertebrate reproductive strategies
73
2.2
Human reproduction
83
2.3.1
Human nervous system
117
2.3.2
Receptors
151
2.4
Human endocrine system
179
2.5
Homeostasis in humans
197
2.6
Plant responses to environment
207
3
Human impact on environment
217
4.1
Evolution
265
4.2
Evolution of humankind
305
4.3
Hominin fossil sites
337
4.4
Alternatives to evolution
357
Bibliography
360
iii
DNA: Code of life
1. Life at molecular, cellular and tissue level
Before studying nucleic acids, it is very important to revise the structure of a typical cell so that you know what cytoplasm is, what ribosomes are and where they are found and how the nucleus is made up.
1.1 DNA: the code of life What is extracellular DNA? Small amounts of DNA are found outside the nucleus in mitochondria in plants and animals and in chloroplasts in plants. This is called extranuclear DNA. See page 64
What are nucleic acids? Nucleic acids have been called the ‘molecules of life’ or the ‘most extraordinary molecules on Earth’ as they have the capacity to store the information that controls cellular activity and the: specialisation of cells to form tissues arrangement of tissues into organs. This enables organisms to perform all the functions necessary to carry out the basic processes of life, ie movement, nutrition, respiration, excretion, growth, reproduction and responding to stimuli. They do this by controlling the synthesis of proteins. Making proteins may seem a far cry from controlling the activities of cells and their development, but it is the first step in that direction. Proteins not only make up much of the structure of the body but, as enzymes are proteins, they also control the chemical processes inside cells. In this way nucleic acids ultimately control the structure and functioning of all living organisms. The two nucleic acids found in cells are: 1. Deoxyribonucleic acid (DNA) 2. Ribonucleic acid (RNA)
Chromosomes and genes What are chromosomes? Chromosomes were discovered by chance in 1888. They absorb dye very easily which is why they were called chromosomes – coloured bodies. This makes them visible under a microscope but they can only be seen as individual threads when a cell is dividing. Chromosomes are long, thin, intertwined thread-like structures made up of a strand of DNA wound around proteins called histones. Some histones are attached to the DNA and help it to coil up during cell division. The DNA molecule is coiled so that these long structures can fit inside the nucleus. There are nearly two metres of DNA squeezed into each human cell.
1. Deoxyribonucleic acid – DNA DNA's discovery has been called the most important biological work of the last 100 years, and the field of genetics may be the scientific frontier for the next 100 years.
DNA molecule wound around a core of histone proteins
A section of a chromosome
Where is DNA found? DNA is found mainly in the nucleus of a cell where it forms an important part of the chromosomes that make up the chromatin network. chromatin = chromosomal material made up of DNA, RNA and histone proteins as found in a nondividing cell
In body (somatic) cells chromosomes occur in homologous pairs. The chromosomes of a homologous pair are the same size and shape and have the same genes in the same place. See page 19.
1
DNA: Code of life
What are genes?
Who won the Nobel prize?
Genes are sections of DNA molecules that control hereditary characteristics, ie they are the basic units of heredity in living organisms. For more information on genes see page 34.
Of the four DNA researchers only Rosalind Franklin had any degrees in chemistry. Through her x-ray diffractions she suspected that all DNA molecules were helical in structure but waited to announce this finding until she had sufficient evidence. She died of cancer aged 37 years before expressing her views. In 1962, when Watson, Crick and Wilkins won the Nobel Prize for physiology/medicine, Franklin had died. The Nobel Prize only goes to living recipients and can only be shared among three winners. Were she alive, would she have been included in the prize?
Who discovered the structure of DNA? At King's College in London in the early 1950’s, Maurice Wilkins and Rosalind Franklin were trying to work out the structure of DNA. They took an experimental approach, looking particularly at X-ray crystallography, ie diffraction images of DNA. At the same time at the Cavendish Institute at Cambridge University, graduate student Francis Crick and research fellow James Watson also became interested in determining the structure of DNA. They analyzed the x-ray data collected by Rosalind Franklin and others. They then built a model out of brass plates and clamps and other bits of laboratory equipment and realized that nucleic acids are arranged on a twisted ladder, with two runners made of phosphates and sugars, and a series of rungs made of pairs of organic compounds known as bases. What about genetic replication? Watson and Crick developed their ideas about genetic replication in a second article in Nature, published on May 30, 1953. The pairing of the bases, ie A=T and C=G suggested that given a sequence of bases in one strand, the other strand was automatically determined. This meant that when the two strands separated, each served as a template for a complementary new chain, ie each strand could replicate.
Learning activity 1 DNA 1.1 Name the three researchers who discovered the structure of DNA and won the Nobel Prize for physiology/medicine in 1962. (3) ________________________________ ________________________________ 1.2 Name the researcher that had a degree in Chemistry but did not win the Nobel Prize. (1) Explain why she did not win it. (1) ________________________________ ________________________________ 2.1. Where is most of the DNA found in a cell? (2)
After the ‘double helix’ model, there were still questions about how DNA directed the synthesis of proteins. In 1961, Francis Crick and Sydney Brenner provided genetic proof that a triplet code was used in reading genetic material in DNA and transferring this information from the nucleus to the cytoplasm via RNA to where proteins are made. The two had shown that in DNA, form is function: the double-stranded molecule could both produce exact copies of itself (replicate) and carry genetic instructions, ie that the sequence of the bases in DNA forms a code by which genetic information can be stored and transmitted.
________________________________ 2.2. Mention two other organelles in which DNA is found. (2) ________________________________ 3. What is: 3.1 chromatin? (2) ________________________________ 3.2 a chromosome? (2) ________________________________ 3.3 a gene? (2) ________________________________ Total [15] 2
DNA: Code of life For example:
The structure of DNA The shape of DNA is rather like a long, twisted ladder, forming a stable, 3-dimensional double helix.
sugar-(deoxyribose) nitrogenous base (thymine)
S
T
P phosphate
A thymine nucleotide
DNA double helix
As there are four different nitrogenous bases, there are four different nucleotides. These four bases are the foundation of the genetic code, instructing cells on how to synthesise enzymes and other proteins. The four nucleotides are the same in all animals and plants. An adenine nucleotide of a human is the same as the adenine nucleotide of a frog or a sunflower etc.
Before you go any further, make sure you know the names of the four bases. How is the double helix made up? See diagram in Learning Activity 2
What are base pairs? The shape and size of the four bases differ so that: Adenine will only bond with thymine (and thymine with adenine) or uracil (See RNA) by means of two hydrogen bonds eg A=T, A=U. Cytosine will only bond with guanine (and guanine with cytosine) by means of three hydrogen bonds eg C G. The hydrogen bonds are weak.
A model of a DNA molecule
Nucleotides The double-stranded DNA molecule is made up of repeating units, building blocks (monomers), called nucleotides, linked together to form long nucleic acids chains, (polymers). Each nucleotide is made up of a:
The outer two strands of the ladder are formed by a chain of alternating sugar/phosphate links. The bonds between the sugar and phosphate molecules are strong. The rungs of the ladder are formed from pairs of bases linked by weak hydrogen bonds. The nitrogenous base pairs are attached to the sugar molecules.
sugar molecule – deoxyribose (S) phosphate molecule (P) nitrogenous base which may be: – adenine (A) – thymine (T) – guanine (G) – cytosine (C)
How are base pairs classified? There are two groups of nitrogenous bases – purines and pyrimidines. Purines are made up of two rings of nitrogen, oxygen and hydrogen atoms. Examples are guanine and adenine.
3
DNA: Code of life
Learning activity 2
Pyridmidines are made up of one ring of similar atoms and are therefore much smaller and uracil (see RNA - page 9). A base pair is always made up of one purine and one pyrimidine.
Structure and importance of DNA Question 1 1.
What shape is a DNA molecule? (1) ________________________________
How do organisms differ? Determining the order of the bases in a DNA chain is known as DNA sequencing. It is this sequence of the four bases that determines the genetic code (genetic information) of an organism. Organisms differ because of the sequence in which the base pairs are strung together. For example, the sequence ACCTGA represents different information than the sequence AGTCCA in the same way that the word ‘post’ has a different meaning from ‘stop’ or ‘pots’, all made up of the same letters. The sequence in certain sections of DNA in a human is different from the same sections in every other human being (except in identical twins) which results in the differences between individuals.
2. What are the monomers called that make up a DNA molecule? (1) ________________________________ 3. Name the sugar found in DNA. (1) ________________________________ 4. Study the symbolic representation of a DNA molecule and write down the labels 1 to 9. (9) 4. 1.
2. 3. 5.
What is the role of DNA? DNA molecules: carry hereditary information in each cell in the form of genes. provide a blueprint for an organism’s growth and development by coding for protein synthesis. (See protein synthesis, page 12) can replicate, ie can make a copy of themselves (see page 7) so that a copy of the genetic information is passed on to each daughter cell formed during cell division. This ensures that the genetic code is passed on from generation to generation.
9.
8.
6.
What is non-coding DNA? Less than 2% of a human DNA actually codes for proteins; the rest consists of non-coding DNA. Protein-coding regions of a DNA molecule are called exons and they are interrupted by the non-coding regions called introns. Complex organisms contain much more of this non-coding DNA than less complex organisms. The non-coding regions were thought to be ‘evolutionary junk’ but they are now known to form functional RNA molecules which have regulatory functions.
1.
2.
3.
4.
5.
6.
7.
8.
9.
4
7.
DNA: Code of life
Learning activity 3
5. Explain how purines are similar to and differ from pyrimidines. Give examples of both groups. (7)
Investigation:
of
DNA
molecules from onion skins
________________________________
Aim:
________________________________
To extract DNA from onion skin cells and examine the threads. The chemical properties of DNA will be used to extract it from onion cells. Instructions
________________________________ ________________________________
If equipment is scarce, this investigation can be done as a demonstration but it would be better to divide the class into groups – the number depending on the number of learners in the class. Once the investigation has been completed, each learner is to answer the questions at the end of the investigation on an A4 sheet which must be handed in. What is needed by each group?
________________________________ ________________________________ ________________________________ ________________________________ [19] Question 2 1.
Extraction
Large metal pot, mixing bowl, an onion, a sharp knife or a pestle and mortar (if available), 250ml cup or beaker, clear dishwashing liquid, salt, coffee filter, funnel, ethanol, thermometer, thin glass rod. Procedure 1. Prepare two water baths
Why is DNA so important? (2 x 3) ________________________________ ________________________________ ________________________________
For the hot water bath a large metal pot can be used with a thermometer to ensure the water stays between 550 – 600C. For the cold water bath, a mixing bowl filled with ice and water kept at around 4°C. 2. Prepare onion Coarsely chop one onion in a pestle and mortar or by hand into a pulpy sludge and tip it into a 250ml cup or beaker. Chopping helps to separate the cells and begins to break down the cell walls.
________________________________ ________________________________ ________________________________ 2. What is non-coding DNA? (2) ________________________________ ________________________________ 3. What percentage of DNA in a cell is noncoding? (1) ________________________________ 4. What role does this non-coding DNA play? (2) ________________________________ ________________________________ [11] Total [30]
Chopped up onion
3. Make a soap/salt solution In a 250ml cup or beaker, dissolve one tablespoon dishwashing liquid and one level 1/4 teaspoon table salt in 100 ml of distilled water. Stir gently to avoid creating foam. 5
DNA: Code of life Continue stirring for a few minutes until the salt is dissolved. The liquid detergent causes the cell membranes to break down so that DNA can be released. The salt (NaCl) will enable nucleic acids to precipitate out of an alcohol solution as it causes the DNA strands to clump together. 4. Pour the solution over the chopped onion
inside of the test tube with the filtrate to create a 1 cm alcohol layer on top of the filtrate. Try to prevent the ethanol and filtrate from mixing. Ethanol dissolves the soluble components of the filtrate forming a solution.
test tube alcohol layer filtrate (onion extract)
9. Precipitation of DNA Leave the solution for 2-3 minutes without shaking it. DNA and salt, being insoluble in ethanol, clump together, forming a white precipitate at the interface between the ethanol and the filtrate. This milky white substance is the DNA of the onion. precipitate = solid material that comes out of solution due to a chemical or physical change
5. Heat in hot bath Place the mixture in the hot water bath for 10 – 15 minutes (no longer). During this time, press the chopped onion mixture against the side of the cup/beaker with the back of the spoon. The heat treatment softens the phospholipids in the cell membranes and denatures the enzymes which, if present, would cut the DNA into small fragments so that it could not be extracted in long strands. 6. Cool in cold bath Cool the mixture in the ice water bath for 5 minutes. Continue pressing the chopped onion mixture against the side of the cup with the back of the spoon. The extracted DNA molecules are fragile and can break apart easily. The cold conditions cause the chemical reactions to take place more slowly so this slows down the rate at which the DNA breaks up. 7. Filter the mixture Filter the mixture through a coffee filter into a clean test tube. This is a slow process. The filtrate consists of dissolved DNA as well as other biochemicals such as RNA and proteins. DNA is a very long molecule but it is small enough to pass through the holes in the filter paper. 8. Add ethanol
DNA precipitating
10. Spooling of DNA Dip a glass rod into the ethanol layer in the test tube, slowly turning it in one direction to spool out the onion’s stringy DNA. spool = wrap around
What is the purpose of this experiment? This simple experiment is an introduction to the procedures of collecting DNA that are used in
Pour the ethanol (alcohol) into a test tube and then chill it by putting it into the cold water bath. Slowly pour the cold ethanol down the 6
DNA: Code of life code is passed on to each daughter cell formed during cell division.
molecular biology, eg for diagnosing disease or genetic disorders. Questions 1. Describe the appearance of the DNA you extracted. (2)
How does replication take place? The process is catalyzed by the enzyme DNA polymerase. The double helix unwinds and then the process of replication takes place in three steps: 1. The weak hydrogen bonds between the two strands break, allowing the strands to part. Like a zip unzipping. Each exposed strand acts as a template.
________________________________ ________________________________ 2. In your own words summarise the main steps you took to extract DNA molecules from onions and the importance of each step. (9) ________________________________
exposed bases
________________________________ ________________________________ ________________________________ ________________________________ Base pairs of DNA separating
________________________________
2. Free nucleotides in the cytoplasm bond to their matching, exposed base partners. The fact that the base pairing is complementary ensures that an exact duplicate of each DNA molecule can be made.
________________________________ ________________________________ 3. Why must the glass rod be rotated in the same direction when spooling DNA? (2) ________________________________ ________________________________ 4. Do you think your results would be different if you were to use a fruit or vegetable other than onions? Explain. (2)
C
________________________________ ________________________________ ________________________________ Total [15]
DNA replication
new nucleo tides added to each strand, forming two complete new strands
DNA replicating itself
Replication is the process of making a new DNA molecule from an existing DNA molecule which is identical to the original molecule. It takes place in the nucleus of a cell during the interphase (in between cell divisions) in the cycle of a cell.
The fact that A will only bond with T and C only with G, makes sure that the sequence of the bases in the daughter DNA is exactly the same as in the parent DNA. One DNA double helix therefore becomes two identical double helices, each containing one old and one newly synthesized strand.
Why is replication necessary? The DNA needs to produce another molecule exactly the same as itself to ensure that the genetic 7
DNA: Code of life 3. What enables the two strands of DNA to separate? (2)
old strand
________________________________ 4. Which DNA strand of the diagram is the new strand, the darker one or the lighter one? (1)
new strand new strand
________________________________ 5.
old strand
Label the four complementary base pairs, 1 – 4. (4) ________________________________
Two new DNA molecules are formed; each identical to the original molecule.
________________________________ ________________________________ ________________________________
3. The two daughter DNA molecules each twist to form a double helix which then winds itself around the histones (proteins), forming a chromosome. The whole procedure only takes a couple of seconds.
[10] Total [14]
Learning activity 4 Replication Question 1 In your own words, explain exactly why replication is so important to the development of an organism. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ [4]
1.
Question 2 The adjacent diagram shows a section of a DNA molecule replicating.
2.
1.
3. 4.
Name the enzyme that catalyses the process of replication. (2) ________________________________
DNA strand replicating
2. When does replication take place? (1) ________________________________
8
DNA: Code of life
DNA profiling/fingerprinting
How is a DNA profile made?
Each person has unique DNA (except for identical twins), despite the fact that 99.9% of human DNA is identical. The differences occur in the noncoding part of DNA. DNA profiling involves the extracting and identifying the highly variable regions of a person’s DNA that contain repeating sequences of base-pairs called STRs (short tandem repeats), eg CAGACAGACAGA is a repeat of CAGA three times. At the same point in the DNA of different people, the number of repeated sequences of base pairs varies considerably, so distinguishing one DNA profile from another. From 13 – 20 different sites on DNA molecules are investigated; enough to show that an individual’s profile is unique. DNA profile = an individual’s unique DNA fragments, separated by electrophoresis
Scientists can use these repeated sequences that vary to generate a DNA profile of an individual, using samples from blood, bone, hair and other body tissues and products. The cells are treated with chemicals to extract the DNA. Restriction enzymes are used to cut at the beginning and end of each repeated sequence, resulting in fragments of different lengths. Through a complicated process known as DNA amplification, large numbers of these fragments are made to provide a substantial amount of DNA to work with. The DNA fragments that result are then separated and detected, using different techniques such as electrophoresis. gel electrophoresis = a method to separate large molecules mainly on the basis of size and electrical charge
9
DNA: Code of life In this way a pattern is obtained that reflects different numbers of base pair repeats in different individuals; the length of a particular DNA fragment depends on the number of repeats present. These separated DNA fragments are represented as dark bands on a piece of film. This is a DNA fingerprint.
DNA profiles of people suspected and convicted of crimes. If a match is made between a sample and a stored profile, the criminal can be identified. 2. Diagnosing inherited disorders DNA profiling provides medical professionals with information needed to determine hereditary diseases. This enables parents to make decisions concerning affected pregnancies and gives them a chance to prepare for proper treatment of an affected child.
Each of our cells carries an identical set of this unique DNA that differs from that of any other person (except in identical twins). If two genetic profiles show identical banding patterns, it is virtually certain that they come from the same person. In related people, some parts may be similar, but no-one else will have exactly the same sequences in every part of their DNA. The following picture shows a set of a family’s DNA fingerprints. Can you see which child has genes (STRs) from which parent? mother
child 1
child 2
3. Identifying casualties If the army kept a set of DNA fingerprints of all soldiers, they could be used to identify unrecognizable casualties. 4. Paternity testing The correct use of DNA profiling can contribute to ruling out clear non-matches in paternity cases. See page 61
father
What are the views against DNA profiling? 1. Violation of privacy Many people believe that the use of DNA profiling to store identifiable information about people is a violation of privacy. This could be harmful, eg the information regarding genetic traits could lead to health insurers denying coverage or claims. 2. Issues on accuracy The accuracy and efficiency of DNA fingerprinting depends on the competency of equipment, laboratory personnel and experience. Possible errors in the procedures inside the laboratory can lead to incorrect information.
How is DNA profiling used? 1. Forensics forensics = the use of different scientific technologies to investigate a crime Identifying differences in the DNA of individuals is very useful in forensic investigations. Because each individual has its own unique genetic sequence, DNA can provide a means of identification accurate enough to be used in the courtroom. Traces of DNA left at a crime scene can often prove to be crucial evidence, eg if stains of blood, skin or semen left on the victim can be matched to DNA fingerprints of the suspect, the criminal has been found. In addition, this technology has reversed convictions and set innocent people free.
3. Manipulation Tampering, irresponsible handling and manipulation of data in genetic profiling, could lead to false information.
Learning activity 5 DNA profiling and forensics Question 1 1. How many different sites on a DNA molecule are usually analysed in constructing a DNA profile? (1) ________________________________
Note: Where there is no suspect for a particular crime, DNA samples collected at the crime scene may be compared with DNA profiles stored on a National DNA Database. This is a resource which contains
2. What makes these sites suitable for DNA profiling? (2) ________________________________ 10
DNA: Code of life
2. Ribonucleic acid – RNA
3. What is a short tandem repeat (STR)? (2) ________________________________
The different forms of RNA are made in nucleus by DNA. Together with DNA, they essential in determining the structure functioning of all living organisms as they involved in protein synthesis.
4. What distinguishes one STR from another? (1) ________________________________ [6] Question 2
What is the structure of RNA? RNA is a single-stranded molecule (polymer) made up of nucleotides. Each nucleotide is made up of a sugar (ribose), phosphate and a nitrogen base. The four nitrogenous bases are adenine (A), uracil (U), cytosine (C) and guanine (G).
victim - Z
evidence - Y
suspect 3
suspect 2
The DNA profiles below are a hypothetical example of a forensic result where the victim, Z, was raped and murdered. There were three suspects in this case and blood samples were taken from each so that fingerprints could be made of their DNA. A semen sample was taken from the body of the victim and used as evidence – Y on the X-ray film. suspect 1
the are and are
S
A
P
P S
U
P S
G
P S
C
RNA strand
1.
What is the function of RNA? RNA carries instructions from DNA in the nucleus to the ribosomes in the cytoplasm of a cell where it controls the synthesis of proteins from amino acids.
Which of the three suspects was probably the killer? (2)
What are the similarities between DNA and RNA? 1. DNA and RNA are both made up of: polymers nucleotides that are made up of a sugar (ribose), phosphate and a nitrogen base four nitrogenous bases 2. They are both responsible for the synthesis of proteins.
________________________________ 2.
Explain how you have come to this conclusion? (3) ________________________________ ________________________________
3. Explain why a sample was also taken from the victim and analysed. (2) ________________________________ ________________________________ 4. Explain the term forensic. (2) ________________________________ ______________________________[9] Total [15]
RNA carries instructions to the ribosomes 11
DNA: Code of life
Learning activity 6 Differences between DNA and RNA Complete the following table that summarises the differences between DNA and RNA molecules. (10) DNA
RNA
Shape of molecules Relative length of molecule Sugar present Bases present Base pairing
Total [10]
Protein synthesis
All cell chemical processes are controlled by enzymes. The synthesis of these protein enzymes is determined and controlled by DNA and RNA.
Are there different types of RNA? There are different types of RNA made in nucleus, each of which has a function in synthesis of proteins. They are: messenger RNA (mRNA) – functions on ribosome transfer RNA (tRNA) – is located in cytoplasm ribosomal RNA (rRNA) – functions on ribosome.
the the
Note that a uracil base (not a thymine base) will pair with an adenine base. A completed strand of mRNA breaks away from the DNA. The DNA then re-zips. The relatively small mRNA moves through the pores of the nuclear membrane and carries the genetic code to the ribosomes which are the sites of protein synthesis.
What determines which protein is made? A protein is a long chain (polymer) of small units (monomers) called amino acids. There are twenty different amino acids that are involved in protein synthesis. These may combine in various numbers in various sequences to form thousands of different proteins. The shortest protein has 50 amino acids.
the the the
A. What happens in the nucleus?
Think of the 26 letters of the alphabet and how many different words can be formed from them. The order in which the amino acids are linked determines what kind of protein is made, eg the protein keratin has a different sequence of amino acids from the protein haemoglobin.
mRNA is formed in the nucleus in the same way as DNA is replicated. The process is called transcription as the coded message in DNA is carried across (transcribed) into the new mRNA molecule, which carries it to the ribosome. Transcription of DNA
What is the role of mRNA? The sequence of amino acids is determined by the instructions from the genetic code in the DNA molecules which is passed on to mRNA. The genetic code is carried as a sequence of ‘codewords’ which are transcribed to the mRNA. Each ‘codeword’ is made up of any three bases and is called a codon, eg cytosine – adenine – guanine. There are 64 different codons and all except three, code for one of the 20 amino acids used to form proteins. Some amino acids are coded for by more than one codon. The three codons that do not code for an amino acid are called stop codons. (UGA, UAA and UAG) This is because there are no tRNAs that have anticodons for these three so no amino acids can be brought to the chain.
Transcription is the process by which DNA makes and codes mRNA. The process starts when a small piece of DNA, a gene, unwinds and the two strands separate (unzip). New nucleotides pair up with their complementary bases on one of the DNA strands. This strand is called the template (pattern) as it carries the code. The nucleotides join up to form a strand of mRNA. The sequence of nucleotides is therefore determined by the sequence of the template DNA nucleotides. In other words the DNA transcribes its genetic code to the mRNA. The genetic code therefore is simply the sequence of some nucleotides in a DNA strand. 12
DNA: Code of life
For example, if the codon on mRNA is GGA, the anti-codon of the tRNA will be CCU. This enables amino acids to link up in the correct sequence.
A codon is written, using the first letter of the different bases. For example, the sequence CCG (cytosine, cytosine, guanine) is the codon for the amino acid glycine and CAG (cytosine, adenine, guanine) is the codon for valine.
mRNA
valine amino acid 1
valine amino acid 2
glycine amino acid 3
C
A
G
C
A
G
C
C
G
G
T
C
G
T
C
G
G
C
You do understand why, don’t you?
The tRNA molecule is released to carry more of its specific amino acid to the ribosome. Catalysed by enzymes, the amino acids link together with peptide bonds to form a polypeptide chain. The polypeptide chains link together to form the final functional protein. To sum up:
DNA 3 bases forming codons, each of which codes for one amino acid
Translation is the process by which a specific protein is formed from a chain of amino acids due to the sequence of codons in the mRNA, which, in turn, was coded by the DNA.
The triplet code of bases is the basis of the genetic code as a gene is made up of a group of codons that code for the synthesis of one protein. The order of codons in mRNA will therefore determine the sequence of the amino acids which will determine which protein is made.
What is the role of rRNA? rRNA is the most common form of RNA in the cell and it, together with proteins, makes up the ribosomes. The rRNA moves from codon to codon along the mRNA, reading the code. rRNA therefore plays an important role in controlling the process of protein synthesis.
B. What happens at the ribosomes? The mRNA binds to the ribosome at the start codon (first codon). The codons of the mRNA act as a template (pattern) that determines the order in which the amino acids are linked.
This is all a bit confusing! Go over the following summary and diagrams very carefully. They should help you to understand the process.
What is the role of tRNA? There are at least 64 different tRNA molecules, made from nucleotides found in the cytoplasm of cells. Each tRNA has three bases at one end called an anti-codon which picks up a specific amino acid found in the cytoplasm and transfers it to a ribosome. The most important feature of tRNA is that it can bind to an amino acid at one end and to mRNA at the other, depositing its amino acid in the correct position to form a specific protein. See the diagram on page 14.
Learning activity 7 Protein synthesis Read the text and very carefully and neatly fill in the relevant labels on the diagrams. Enjoy thinking! 1. The mRNA, with its codons, moves through the pores of the nuclear membrane into the cytoplasm where it binds with a ribosome. 1. _________________ in 2. ________________ membrane
Translation of RNA into proteins One of the codons, the 'start signal', begins the process of making a protein from amino acids. Three of these codons act as 'stop signals' that indicate that the message is over and the protein chain is complete. All the other codons code for specific amino acids. The anticodon bases link up to their complementary bases of the codon. This process is called translation, as the code on the mRNA is translated into a sequence of amino acids.
3. _________________ 4. _________________
5. A single _________________
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